In order to increase data throughput without the installation of new fiberoptic links, Wavelength Division Multiplexing (WDM) systems are being deployed. These systems rely on the tunability of semiconductor lasers to access a wider portion of the spectrum that optical fibers propagate. This can result in an increase in data rates by an order of magnitude or more.
Standards have been promulgated for the channel spacings in these WDM systems. The frequency spacings are tight for optical frequencies. For laser diodes operating at around 1.5 micrometers (.mu.m), it is typically 100 gigahertz (GHz); this translates to an approximately 0.8 nanometers (nm) minimum wavelength channel spacing.
How the laser diodes are tuned to operate in the various channels of the WDM systems depends on the specific types of laser diodes used. The wavelength of distributed feedback (DFB) laser diodes is tuned by changing the temperature of the diodes. Temperature control is typically implemented in the context of laser communication modules with a thermo-electric cooler. These devices extract heat using Peltier effect in a manner that can be electrically modulated. Fabry-Perot lasers are similarly tuned by controlling their temperature. Distributed Bragg (DBR) lasers as tuned by temperature and injection current.
Solutions have been proposed for maintaining the proper channel spacing in these WDM systems. The proposals typically rely on precise factory calibration, due to inherent manufacturing variability in distributed feedback lasers, for example, and manual fine tuning of the WDM module wavelength after installation to detect and adjust for wavelength shifts from aging and environmental effects.